The invention concerns a production system for automatic casting of a hollow body, in particular of concrete, and comprising an inner mould which may be stationary or rise vertically during casting; an outer mould which can be displaced vertically up and down between a lower casting position and an upper free position; a bottom ring for supporting the hollow body and forming a mould part for the lower end part of the hollow body; a top ring for forming a mould part for the upper end part of the hollow body; and a gripper for positioning the top ring in the casting position and retaining it during casting.
Today concrete pipes and other hollow bodies to be manufactured in large numbers with a uniform and constant quality are generally cast in automatically operating production systems of the above-mentioned type. A typical system comprises a mould machine having functions for vertically displacing the outer mould and usually also the inner mould or the core vertically up and down. The machine moreover has a table on which the bottom ring, which simultaneously serves as a pallet for the finished pipes, is placed at the beginning of a production cycle. When the outer mould has been lowered and stands on the bottom ring, the mould is filled from above with fresh concrete. In those cases where a vertically displaceable inner mould is used, this is simultaneously caused to rise from below so as to successively define a ring gap between the two mould parts for forming of the pipe wall. During this casting process the concrete is subjected to vibrations by means of one or more vibrators, which are present in the inner mould in most cases.
In a very widely used method the upper end or spigot end of the hollow body is formed by pressing a profile ring down against the upper side of the concrete when the mould has been filled and the inner mould is present in its upper position. After this operation stripping of the pipe is initiated, the inner mould being pulled down and the outer mould up. During this operation the profile ring remains in its lower pressing position to prevent the pipe from being pulled apart completely or partly by the rather considerable, upwardly directed friction forces to which the outer mould subjects the pipe. Then the profile ring is lifted clear of the pipe, which can now be removed by means of a transport carriage or crane which drives the pipe out to a location for setting.
During setting the pipe remains standing on the bottom ring, which, as previously mentioned, forms a mould part for the lower end or socket end of the pipe, which thus retains its shape and the prescribed tolerances with certainty. Since the profile ring is not carried along, but remains in the machine, the shape of the spigot end, on the other hand, will not be retained during the transport and the setting process without means being provided for this purpose. It has been found that such free spigot ends on newly cast pipes tend to become oval or collapse before the concrete has obtained a sufficient stability and strength. To ensure the dimensional stability of the spigot end as well, a separate top ring is therefore usually placed on the spigot end immediately after casting. This top ring, which is frequently of plastics, can advantageously have walls which support the spigot end interiorly as well as exteriorly. When the pipe, e.g. after one day, has set sufficiently, the top ring is removed again.
These top rings require performance of not insignificant manual labour in the otherwise fully automatically proceeding process. To this should be added that the positioning of the top rings on the spigot ends involves a serious risk, since the spigot end can easily be damaged during this operation if extreme care is not shown.
To avoid these drawbacks, a known automatic production system employs loose top rings, which are of steel and form a mould part for the spigot ends during casting in the normal manner. Instead of being pulled clear of the finished pipe, as before, the top ring is now left on the spigot end and remains on it until the concrete has set sufficiently. Then the top ring is knocked off and is used again in a subsequent production cycle.
For also the operations necessary for this purpose to be performed automatically, a gripper is incorporated in the production system for fetching the top ring from e.g. a magazine, positioning it in the casting position and retaining it there during casting. This gripper has a central opening through which the mould can be filled with fresh concrete. The gripper is moreover provided with a plurality of claws for gripping some corresponding, upwardly directed gripping parts which are secured on the top side of the top ring. Each claw is activated by a pneumatic or hydraulic drive cylinder, which extends radially outwardly from the circumference of the gripper. This structure entails that the extent of the gripper transversely to its axis will exceed the internal diameter of the outer mould at any rate at the drive cylinders. Because of the size of the gripper it must be positioned above the outer mould during casting. For the same reason the outer mould cannot pass upwards around the gripper during stripping, and the gripper must therefore necessarily first be removed from the path of the outer mould.
This entails that the gripper is not capable of providing any support for the upper end of the pipe during the very critical stripping of the outer mould. In practice, the method can therefore only be used for e.g. well rings having a small height and a large thickness, while slender pipes, which may e.g. have a length of 3000 mm, a diameter of 300 mm and a thickness of the shank of 65 mm, cannot be cast in this manner. In this case, the frictional force involved in the stripping of the outer mould would be greater than the tensile forces which the non-set, newly cast concrete could stand.
Accordingly, there is a need for a new and improved automatically operating production system of the type mentioned in the opening paragraph for casting of hollow bodies, such as concrete pipes, which, via a loose top ring, is adapted to provide support for a hollow casting during stripping of the outer mould and to allow the top ring to remain in unchanged position on the casting until this has set sufficiently on the setting location.